First (probably obvious, but should be verified anyway): are you doing that on a TOS version that is supposed to support Maddalt() at all?
Do you check the Maddalt() return value (should be zero, otherwise something went wrong)?
Second (probably also obvious, but anyway): it should be clear that adding a certain block of Alternate RAM will only work once per session (i.e. should be done once at boot from an AUTO folder program and nowhere else).
Madalt() might (not entirely sure though) need to allocate memory in ST-RAM for its own purpose. If you do not leave enough room (Mshrink()) for the OS to allocate it while your program runs, Maddalt() will probably fail. Make sure you release unused memory immediately after start (as all clean code should do).
Setting up the _FRB cookie (or the Fast RAM buffer itself, respectively) is something different: as this requires permanent allocation of a buffer in ST-RAM, this can be done reliably from a TSR only (i.e. you need to call Mshrink() and Ptermres() at the end of your program instead of doing the "normal" Pterm() or Pterm0() exit). The buffer where your cookie points to needs to stay resident/reserved after the program terminates, otherwise *very* strange things will happen.
If these hints do not help, you should probably show your code.